Field of the Invention
The present invention relates to a hydraulic source control device, and an injection molding apparatus including the hydraulic source control device.
Priority is claimed on Japanese Patent Application No. 2012-178566 filed on Aug. 10, 2012, the contents of which are incorporated herein by reference.
Description of Related Art
In recent years, in apparatuses, such as an injection molding machine and a press forming machine, in order to reduce the energy loss when operating respective control elements or to perform high-precision control, an electric actuator including a servo motor or the like may be used instead of a hydraulic circuit using a working fluid, such as an oil. However, if such an electric actuator is used, there are controlled objects in which control is complicated or the number of parts increases. Therefore, not the electric actuator but the hydraulic circuit is continuously utilized for a controlled object in which control is easier if the hydraulic circuit is used.
In a case where a control element is driven using the hydraulic circuit, generally, a fixed capacity type hydraulic pump is driven by an electric motor. As the electric motor that drives the hydraulic pump, for example, a three-phase induction motor is known. In a case where this three-phase induction motor is used, the flow rate of a working fluid to be discharged also becomes constant because rotational speed is kept constant. Therefore, an extra working fluid exceeding a flow rate that is required for holding a predetermined fluid pressure, is sequentially discharged from a relief valve or the like, and an energy loss is caused by an amount equivalent to this discharged working fluid.
Thus, PCT International Publication No. WO2007/114339 suggests that a variable speed motor having a variable rotational speed be used instead of the three-phase induction motor, and the variable speed motor be controlled so that the flow rate and fluid pressure of a hydraulic pump become command values. In a case where the variable speed motor is used, if the rotational speed is reduced when holding the fluid pressure, it is possible to reduce the discharge amount of the working fluid by a corresponding amount to reduce an energy loss.
On the other hand, in a case where the variable speed motor is used, the flow rate of the hydraulic pump is set by setting a required rotational speed. However, since the leakage amount between fluid chambers bisected by a piston head of a hydraulic cylinder or the leakage amount of a hydraulic valve part or the like is not always constant, the rotational speed of the hydraulic pump is set so that the flow rate of the hydraulic pump becomes a desired flow rate at which the hydraulic cylinder is operated, a flow rate to which the maximum value of the leakage amount of a working fluid that is assumed or obtained by experiments is added, or a flow rate higher than that. Therefore, although the flow rate of an extra working fluid discharged from the relief valve or the like itself is reduced, the working fluid discharged without being utilized is still present, and becomes an energy loss.
Japanese Unexamined Patent Application, First Publication No. 2002-225103 suggests performing feedback control of the rotating speed of a hydraulic pump using a fluid pressure detected by a fluid pressure sensor, in order to obtain the actual fluid pressure of a working fluid supplied to a mold clamping cylinder as a desired fluid pressure. In a case where feedback control is performed by such a fluid pressure sensor, it is possible to discharge a working fluid of an amount equivalent to a leakage amount from the hydraulic pump without excess or deficiency, and it is possible to hold the working fluid at a desired fluid pressure.
However, in a case where the variable speed motor is used, the feedback control of the rotational speed is performed based on a detection result of the fluid pressure sensor. Therefore, there is a possibility that control may oscillate and fluid pressure may not be stable in the case of a complicated drive system that is easily disturbed, and there is a problem in that control becomes complicated in a case where an attempt to suppress oscillation is made. Additionally, since the fluid pressure sensor or the like should be added in order to perform the feedback control, there is a problem in that the number of parts will increase and lead to cost increase. Additionally, since the feedback control is performed, for example, in a case where the fluid pressure has overshot a predetermined value, the hydraulic pump should be reversely rotated immediately to cause the working fluid to flow back to the hydraulic circuit to lower the fluid pressure. However, since the fluid pressure is reversed from a positive pressure to a negative pressure at this time, and an inflection point where flow is reversed and the fluid pressure is zero is crossed, supply control of the working fluid is apt to become unstable.